We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on d 13 C of the fast-turn-over carbon pool (water soluble organic carbon and CO 2 emitted from heterotrophic tissues), including their diel variation, along the pathway of carbon transport from the foliage to the base of the stem.For that purpose, we determined d 13 C in total and water-soluble organic matter of the foliage plus d 13 C and d 18 O in phloem organic matter of twigs and at three heights along the stem of Pinus sylvestris over a nine-day period, including four measurements per day. These data were related to meteorological and photosynthesis parameters and to the d 13 C of stem-emitted CO 2 .In the canopy (foliage and twigs), the d 13 C of soluble organic matter varied diurnally with amplitudes of up to 1.9%. The greatest 13 C enrichment was recorded during the night/early morning, indicating a strong influence of starch storage and remobilization on the carbon isotope signatures of sugars exported from the leaves. 13 C enrichment of soluble organic matter from the leaves to the twig phloem and further on to the phloem of the stem was supposed to be a result of carbon isotope fractionation associated with metabolic processes in the source and sink tissues. CO 2 emitted from the stem was enriched by 2.3-5.2% compared with phloem organic matter. When day-to-day variation was addressed, water-soluble leaf d 13 C and twig phloem d 18 O were strongly influenced by c i /c a and stomatal conductance (G s ), respectively.These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence d 13 C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the d 13 C of respired CO 2 must be considered when using the latter for partitioning of ecosystem CO 2 fluxes or when the assessment of d 13 C in organic matter is applied to estimate environmental effects in c i /c a .
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.